Development of Asymmetric Polycarbonate (PC) Membrane for Carbon Dioxide Removal from Methane

The morphology of asymmetric membrane strongly influences the performance of
membrane in removing C02 from CH4. The understanding of mechanism of
asymmetric membrane formation is very crucial in order to produce desirable
morphology that leads to enhancement of the membrane performance. The objectives
of this work are to study the effect of various preparation conditions on the
morphologies of asymmetric polycarbonate (PC) membrane and its relation to
CO2/CH4 separation characteristic.
Asymmetric PC membranes were fabricated using dry/wet phase inversion technique.
The effects of solvent - non-solvent pair, non-solvent concentration, evaporation time
and composition of water-MeOH mixtures in coagulation bath on membrane
morphologies were investigated. The mechanism of membrane morphologies
formation was explained using solvent and non-solvent evaporation, solubility
parameter and coagulation value. Dichloromethane (DCM) and chloroform were
selected as more volatile solvents while ethanol (EtOH), propanol (PrOH) and butanol
(BuOH) were used as non-solvents. In addition, methanol (MeOH) and 1,1,2
trichlorethane (TEC) were used as the coagulant and less volatile solvent,
respectively. Membrane characterization was carried out by using SEM and DMA.
Gas permeation unitwasusedto evaluate the performance of membrane.
Experimental results showed that high boiling point of BuOH was responsible in
forming highly porous substructure with macrovoid formation in the DCM and
chloroform-based membranes prepared using BuOH as non-solvent. Increasing BuOH
concentration from 0 to 10 wt.% in DCMcasting solution produced macrovoids and a
more porous substructure. This is due to smaller coagulation value and solubility
parameter difference between solvent mixtures and MeOH. hi addition, by increasing
the evaporation time for casting film from 0 to 60 seconds, and water content from 0
vol.% to 30 vol.% in MeOH coagulation bath, less porous and macrovoid-free
substructure were obtained. This is due to thicker skin layer formation and larger
solubility parameter difference between solvent mixtures and MeOH, respectively.
The performance of asymmetric PC membranes was evaluated bymeasuring C02 and
CH4 permeances as well as C02/CH4 ideal selectivity. The results showed that C02
and CH4 were strongly dependent upon membrane morphologies formed during
fabrication. Highly porous membranes prepared from DCM-BuOH and chloroform-
BuOH pairs were found to give higher C02 and CH4 permeance as compared to EtOH
and PrOH membranes. Increasing the BuOH concentration from 0% to 10 wt.% of
casting solution would increase the C02 and CH4 permeances as a result from highly
porous substructure and the existence of macrovoids. Lower C02 and CH4
permeances obtained on asymmetric PC membranes prepared from the effect of
longer evaporation time of casting film and from the effect of higher water
concentration in MeOH coagulation bath were due to less porous substructure
formation. In term of selectivity, the highest C02/CH4 ideal selectivity of the
fabricated asymmetric PC membrane is approximately 175. These results
demonstrated significant improvement in C02/CH4 separation as compared to other
membranes reported by previous researchers. In conclusion, asymmetric PC
membranes produced in this work show promising performance and have high
potential to be used for C02/CH4 separation.